Active Rockslides in Switzerland – Understanding Mechanisms and Processes
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LANDSLIDES – Causes, Impacts and Countermeasures 17-21 June 2001 Davos, Switzerland pp. 25-34 ACTIVE ROCKSLIDES IN SWITZERLAND – UNDERSTANDING MECHANISMS AND PROCESSES Erik Eberhardt, Engineering Geology, ETH Zurich, Switzerland Heike Willenberg, Engineering Geology, ETH Zurich, Switzerland Simon Loew, Engineering Geology, ETH Zurich, Switzerland Hansruedi Maurer, Applied and Environmental Geophysics, ETH Zurich, Switzerland ABSTRACT To date, the study of rockslides in Switzerland has been largely descriptive and qualitative. Early warning systems are primarily based on displacement measurement systems and the analysis of the rock mass velocity evolution. This paper describes the initial stages of a project focussing on the understanding of rockslide processes and mechanisms that contribute to instability and catastrophic failure. Several examples of unstable slopes in the Swiss Alps are presented and discussed in terms of their geological controls, potential triggering mechanisms and failure evolution. INTRODUCTION Despite improvements in recognition, prediction and mitigative measures, landslides still extract a heavy social, economical and environmental toll in Switzerland. Recent landslides experienced in the Swiss Alps demonstrate the need for a deeper understanding of the geological and physical processes leading to catastrophic slope failure. Large-scale rockslides (e.g. Randa, Sandalp, Goldau, Elm, etc.) illustrate the destructive potential of these mass movements and the need for further study to improve our comprehension of the mechanisms involved. Advances in rockslide hazard assessment and forecasting will only be made possible when the mechanisms responsible for the evolution of catastrophic failures are better understood. A study was therefore initiated in June 2000 to investigate the evolutionary failure processes leading to larger-scale mass movements in massive rock slopes. The working hypothesis of the project contends that rock slope instability occurs through a process that involves the progressive development of a failure surface as opposed to sliding along a pre-existing one. For example, in a massive brittle rock mass, the persistence of most discontinuities is usually limited and shear failure must proceed along a path that passes through existing discontinuities and intact rock bridges. Accordingly, these developments must involve a complex process of fracture nucleation, propagation and coalescence, combined with spatio/temporal variations in pore pressures. This paper presents the results from the first phase of this Swiss-based study, and includes: · a review of progressive failure and the need for mechanistic-based slope stability analysis; · a survey of potential study sites, encompassing several known active rockslides in Switzerland; and · the detailed engineering geology investigation and passive seismic monitoring of three key sites focussing on the acting mechanisms and processes. An overview is then presented of the detailed in situ monitoring program to be commenced in the summer of 2001. This second study phase will involve comprehensive drilling, borehole testing and instrumentation installation focussing on 3-D displacement fields, microseismicity, fracture patterns and the temporal and spatial evolution of fluid pressures. PROGRESSIVE FAILURE - CONCEPTUALIZATION Rockslide studies that do focus on some quantitative aspect of large-scale mass movements are often limited to individual mechanisms or triggering processes. Traditional treatments have pursued phenomenological based approaches where a two-dimensional slide plane is assumed or delineated from survey or air photo data, and a back analysis is performed to determine the limiting equilibrium conditions existing at failure. In other words, the analysis of unstable rock slopes has largely focused on the back analysis of stability along a fully developed failure plane, without considering how the failure plane evolved. The need to consider failure plane evolution as opposed to focussing on phenomenological approaches is underlined by the questions, “Why did the slope fail now? Why did the slope change from an apparently stable condition to that of catastrophic failure”? These are important questions to contemplate if any serious attempts at landslide prediction are to be made, i.e. the question of time. Limit equilibrium analysis and other phenomenological approaches only provide a snapshot of the conditions at the instantaneous moment of failure, and as such they provide a simplified answer as to why the slope failed, but not within the context of time as to “why now?”. To answer the time question, in the framework of a conceptual model, requires the consideration of two factors that form the basis of most rock slope failure analyses. These are the initial slope conditions, or system processes, and the acting triggering mechanisms (Fig. 1). The initial slope conditions often involve, and in the case of most slope analysis methodologies require, the predefinition of the failure surface geometry as a continuous plane and/or as a series of interconnected planes (Fig. 2). It is unlikely, however, that such a network of natural joints forming a complete three-dimensional outline of the eventual failure surface exists prior to failure. The exception being in the case of excavated slopes where a daylighting set of discontinuities may be exposed during excavation allowing kinematic feasibility. In the case of natural rock slopes, consideration must be given to the fact that the slope has remained relatively stable over the past several thousand years with few major external changes occurring with respect to its kinematic state (e.g. loss of confinement along valley walls during glacial retreat). This is not to say that a system of natural discontinuities may not be interconnected forming a significant portion of what will eventually be the failure plane, but that a component of strength degradation with time must occur within the system. For example, the system may incorporate time-based elements such as material creep, fracture propagation, stress corrosion and/or weathering, which in turn act to reduce with time the strength of asperities between locked joint surfaces and/or intact rock bridges situated between natural discontinuities (Fig. 2). The second factor to consider involves the influence of triggering mechanisms. Slope failures typically coincide with known triggering mechanisms such as heavy precipitation and/or seismic activity. However, with the exception of extreme triggering episodes, as in the case of major earthquakes where the frequency of occurrence is on the time scale of hundreds to thousands of years, most triggering episodes do not stand out as being exceptional when compared to those that had occurred in the recent past. In the case of heavy precipitation or high pore pressures, for example, failure rarely occurs during episodes that had not been surpassed previously in the history of the slope in question. CAUSE EFFECT system mass processes trigger movement geological precipitation rock slide mechanical earthquake avalanche hydrological climate mud flow geomorphological land use rock fall biological time debris flow A A ’ B a % instability type I B ’ A B b % instability type II Figure 1. Illustration of cause, involving the interrelationship between system processes and triggering mechanisms, and their effect on inducing mass movements. Figure 2. Continuous sliding surface assumed in simplified rock slope stability analysis (LEFT); rock slope with discrete natural discontinuities and intact rock bridges (RIGHT). Figures 3 and 4 provide an illustration of this point using the 1991 Randa rockslide, which occurred in south central Switzerland. The failure occurred within a massive gneissic rock mass cut by extensive relief joints, parallel to the surface, cross-cut by faults (1, 2). The slide occurred in two stages with the first slide occurring on April 18, 1991 and the second failing on May 9, 1991. Although no clear triggering mechanism could be resolved from the seismic and precipitation records, it was noted that failure coincided with a period of heavy snowmelt (1). This can be seen in the snow height and temperature records for the Zermatt weather station (Fig. 4), located 10 km south of Randa. However, it can also be seen that this was not an exceptional event and that heavier snowmelts had been recorded in previous years. Again, a similar argument can then be made that if the slope did not fail during previous periods of heavy snow melt, why did it fail specifically on April 18, 1991? Figure 3. Schematic cross-section showing the 1991 Randa rockslide (after (3)). Figure 4. Snow depth, temperature and precipitation records for Zermatt climate station near the Randa rockslide (data provided by MeteoSchweiz). Conceptually then, rock slope failure can be viewed as the progressive accumulation of events with time that act to degrade the equilibrium state of the slope, with each event bringing the slope nearer to failure (Fig. 5). In other words, each triggering type episode, such as a heavy rainfall or freeze-thaw cycle, can be viewed as progressively reducing the effective strength or cohesion of the rock mass (e.g. intact rock bridges) until the last triggering episode provides the proverbial “straw that broke the camel’s back” and failure occurs. Thus it is the time factor that contributes to the development of the sliding surface in natural slopes by means of progressive failure. Figure 5. Conceptual illustration